1. Field of the Invention
The invention relates to a voltage controlled oscillator, and more particularly to a voltage controlled oscillator with a transformer.
2. Description of the Related Art
The voltage controlled oscillator is a component used as a local oscillator in many wireless communication devices. With a voltage controlled oscillator circuit, a desired frequency can be easily obtained by adjusting the charging and discharging times of capacitors or capacitive elements. For this reason, a voltage-controlled oscillator circuit is normally used in an apparatus which requires different clock frequencies. Today, CMOS integrated circuits of low power consumption have become widespread. With this trend, various voltage controlled oscillator circuits of the CMOS configuration are being developed. However, an LC tank voltage controlled oscillator usually comprises a large capacitor and a small inductor. Meanwhile, parasitic capacitor effect increases due to the small inductor, resulting in phase noise in the output clock signal of the voltage controlled oscillator.
FIG. 1 is a circuit diagram of an LC oscillator. The inductor L1 placed in parallel with a capacitor C1 resonates at a frequency ω=1/√{square root over (L1C1)}. At this frequency, the impedances of the inductor and the capacitor are equal and opposite. However, in practice, inductors or/and capacitors suffer from resistive components. For example, the series resistance of the metal wire in the inductor can be modeled as the resistor Rs shown in FIG. 1. An infinite quality factor Q of the inductor L1 is defined as L1ω/Rs. In the FIG. 1, the equivalent impedance is given by
                                                        Z              eq                        ⁡                          (              s              )                                =                                                    R                s                            +                                                L                  1                                ⁢                s                                                    1              +                                                L                  1                                ⁢                                  C                  1                                ⁢                                  s                  2                                            +                                                R                  s                                ⁢                                  C                  1                                ⁢                s                                                    ,                            (                  eq          .                                          ⁢          1                )            
and hence,
                                                                                    Z                eq                            ⁡                              (                                  s                  =                  jω                                )                                                          2                =                                                            R                s                2                            +                                                L                  1                  2                                ⁢                                  ω                  2                                                                                                      (                                      1                    -                                                                  L                        1                                            ⁢                                              C                        1                                            ⁢                                              ω                        2                                                                              )                                2                            +                                                R                  s                  2                                ⁢                                  C                  1                  2                                ⁢                                  ω                  2                                                              .                                    (                  eq          .                                          ⁢          2                )            
The magnitude of Zeq in eq. 2 reaches to a peak value in the vicinity of ω=1/√{square root over (L1C1)}, but the actual resonance frequency is still partially dependent on Rs.
According to the described, controlling the resonance frequency by adjusting the capacitor C1 and inductor L1 should be achievable. However, in practice, the inductance of the inductor L1 is not easily precisely adjusted, and the capacitor C1 may suffer parasitic capacitor effect. Thus, the resonance frequency ω is unstable.